In industry, air remains the oxidant, that is to say the combustion oxidizer, most often used.
However, it is known to use oxygen instead of air as oxidant, in particular for improving the efficiency of the combustion and thus the energy yield of an industrial installation comprising a combustion chamber, in order to maximize the use of the thermal energy generated in the combustion chamber and in order to reduce the polluting emissions of the combustion chamber, such as NOx.
While the energy and environmental advantage of combustion with oxygen is well known and recognized, the cost of oxygen in comparison with that of air continues to limit the use of oxygen as oxidant in an industrial context.
In order to improve still more the efficiency of oxycombustion and to thus reduce the need for fuel and for oxygen for a given industrial process, processes for the preheating of oxygen have been developed.
It is known in particular to preheat oxygen in a heat exchanger.
In particular, a method is known for the indirect preheating of oxygen with the residual heat present in the combustion gases (flue gases) at the outlet of the combustion chamber. According to this method, an auxiliary fluid is preheated in an auxiliary exchanger by direct exchange with the hot flue gases. The oxygen is subsequently preheated in at least one main heat exchanger by direct exchange with the hot auxiliary fluid resulting from the auxiliary exchanger. If appropriate, the fuel is also preheated by direct exchange with the hot auxiliary fluid in an additional heat exchanger. Various embodiments of this method are described in particular in EP-A-0 872 690 and WO 2006/054015.
EP-A-2 546 204 describes a glass melting process using another embodiment of the indirect preheating method. During a first phase of the process which is a subject matter of EP-A-2 546 204, a first flow rate DO1 of an oxygen-rich oxidant is provided for the combustion of a fuel in a combustion chamber, this oxygen-rich oxidizer having been preheated by heat exchange with an oxidizer poorer in oxygen, said poorer oxidizer being heated by heat exchange with the flue gases generated in the combustion chamber. During a second phase, use is made, as oxidant, of a reduced flow rate DO2 of the oxygen-rich oxidizer and also of a flow rate DA2 of the heated poorer oxidizer.
EP-A-1 338 848 describes a process for the recovery of heat from the flue gases from a furnace using an oxidant enriched in oxygen. The discharged flue gases are used for the direct or indirect preheating of the fuel and/or of the oxidant in a direct or indirect heat exchange system. The flue gases are subsequently introduced into a heat recovery boiler for the production of mechanical energy. The direct or indirect heat exchange system or systems can be provided with a bypass for flue gases in order to regulate the portion of said flue gases actually introduced into the heat exchange system.
In comparison with other known methods for the preheating of oxygen, the indirect preheating method exhibits the major advantage of greater safety. This is because, in the event of a perforation by corrosion or by erosion inside the auxiliary exchanger, the hot flue gases, liable to contain residual combustible matter, come into contact only with the auxiliary fluid. Similarly, in the event of a perforation inside a main or additional exchanger, the oxygen or respectively the fuel comes into contact only with the auxiliary fluid.
The energy balance of this method is particularly positive.
However, one disadvantage of this method is the space required by the different exchangers and in particular by the auxiliary exchanger, it being known that, in order to avoid heat losses, said exchangers are positioned as close as possible to the outlet of the flue gases from the combustion chamber.
Another disadvantage is the cost of the various heat exchangers which, given the temperatures and the nature of the fluids circulating inside, have to be particularly robust.
The exchangers are designed and sized for optimum operation under the conditions (temperatures and flow rates of the fluids circulating in the exchangers) which correspond to the nominal conditions of the industrial installation comprising the combustion chamber, that is to say to the normal operating conditions of the industrial installation.
However, industrial installations can be induced to operate outside their nominal conditions and more particularly with a higher heat requirement than during their normal operation, for example because of aging or wear of the installation or during an increased output campaign of a melting furnace.
In this case, the operation of the preheating method is not optimal owing to the fact that the auxiliary exchanger exhibits an ability to recover residual heat from the discharged flue gases which is too low with respect to the heat requirement: the residual heat actually recovered is then insufficient for the use or the different uses which it is desired to make of it, such as the preheating of oxygen, the preheating of the fuel and/or other uses. This results in a greater energy consumption and in particular a greater fuel consumption.
The use might be envisaged of an auxiliary exchanger having an ability to recover heat which is greater than that corresponding to the nominal operation of the installation or also the installation might be envisaged of a supplementary auxiliary exchanger to be used only when the need for recovered heat is greater than in nominal operation of the installation. However, this will result in a space requirement and an even greater cost of the installation.
The aim of the present invention is to at least partially overcome the problems described above.